# Deducing the hour based on RA

I'm making a technological project. The goal is to write a program that gets an image of the nightsky and predicts the hour in which the photo was taken.

I implemented a well-known method using the big dipper, but it's not enough. I don't want to limit my program to only work when the big dipper is in the image.

When I read about right ascension, my gut told me seeing a star in the sky and knowing its right ascension (RA) would help me deduce how many hours ago the sunset happened, though I couldn't explain how.

I'm not comfortable enough with this coordinate system, so I need help answering this question: Say I konw the location a photo was taken, and I can identify the celestial objects in the sky and get their RA, can I make some calculations to konw/approximate the hour? If so, how?

• What other information is included in the photograph? Since a camera can be tilted at any angle, you cannot depend on knowing what direction is "up" just based on the "bottom edge" of the photo (unless you want to make that a requirement). In other words, the photo needs some reference, such as the horizon, to determine which direction is "up". Then you could approximate the time by knowing the orientation of the constellation in the photo. You also need the date and latitude since those are the other variables that lead to an answer about the time. Commented Mar 31, 2020 at 22:53
• And of course a program that needs the Big Dipper is going to be useless for those of us who live in the southern hemisphere. ;) Commented Mar 31, 2020 at 23:12
• Does this answer your question? Is it possible to determine an observer's position on Earth from a photo of the night sky and the time of the shot? Commented Apr 1, 2020 at 7:10
• Hi, your information was relatively helpful, especially konwing that it is possible. About tilted cameras - for now we require the photos to be aligned, though we might make an app for this which will save this angle using the gyro device on the phone. John, you said konwing the date and latitude I can get the answer. Do you know how will I do that? Can you write the equations down or refer me to related sources? Thanks! Commented Apr 1, 2020 at 8:46
• Hi, thanks again for the reply. For now let's put aside the engeering challenges such as taking into account the distortion of the photograph, there are algorithms for this. For now I just want to know what are these back-calculations I need to make to get the time of the photo. Commented Apr 1, 2020 at 22:50